This application claims the priority of British Application No. 0116812.9 filed Jul. 9, 2001, the disclosure of which is being incorporated herein by reference.
The present invention relates to an off-set elimination system for a vibrating gyroscope, and more particularly relates to an off-set elimination system for use with micro-machined gyroscopes.
It has been proposed previously to provide various types of micro-machined gyroscopes. A gyroscope of this type is described in WO99/38016. In this specification a gyroscopic sensor is provided which is formed from a planar substrate, which is an etched silicon substrate. The substrate is etched to define a beam, the opposed ends of which are adapted to be fixed in position, the beam having a preferential bending direction which makes an acute angle with the plane of the planar substrate. The beam carries an inertia mass comprising two arms or plates which are interconnected by a connecting bar, the central part of the connecting bar being formed unitarily with part of the beam. The bar extends substantially at 90xc2x0 to the beam, and the two arms or plates are on opposite sides of the beam. Capacitative plates are located adjacent areas of a conductive layer formed on the arms, and the capacitative plates are provided with potentials or an excitation voltage which cause the inertia mass to rotate about the axis of the connecting bar with a xe2x80x9csee-sawxe2x80x9d action which, because of the configuration of the beam, leads to a rotational oscillation of the inertia mass in the plane of the substrate. If the arrangement is then rotated about an axis coincident with the connecting bar, the inertia mass rotates about an axis coincident with the beam with an oscillating rotation. This is detected by further capacitative plates, on which a detection voltage appears, to determine the angular velocity of the rotation of the arrangement about the axis coincident with the beam. The angular velocity of the entire gyroscope about the axis coincident with the connecting bar can then be calculated.
Other micro-machined vibrating gyroscopes have also been proposed previously.
It has been proposed that such vibrating gyroscopes could be used as sensors in motor vehicles to sense accidents, and in particular to sense a roll-over situation. In such a case the sensing axis, in the following description defined as the xe2x80x9cyxe2x80x9d axis, should be aligned with the longitudinal axis or driving direction of the vehicle. However, it is envisaged that such gyroscopic sensors may also be utilised for measuring the orientation of the car about a vertical axis, which may be of particular use in connection with a navigation system. In such a case the measured angular velocity has to be measured extremely accurately, since angular velocity will be integrated to give a signal equal to the total angle of rotation, and a very small error of the angular velocity will, when integrated, gradually lead to a very big error of the total angle of rotation. In such a case it is very important to eliminate all possible errors.
It has been found that, due to production variations, a large number of undesired sources of error may be found in any typical micro-machined gyroscope which will cause offset errors for the gyroscope. Such errors may be relatively large, especially when compared with the desired resolution for the gyroscope.
For example the spring or spring forces provided effectively by the beam of a gyroscope of the type described above may not be symmetrical, due to a lack of symmetry in the beam caused during manufacture. The result of this can be asymmetrical spring forces which generate a torque that excites the detecting mode of the gyro, simply in response to the potentials which are intended to cause the inertia mass to rotate about the axis of the connecting bar.
The distance or position of the detection and excitation electrodes may be unsymmetrical and the mass or masses of the various arms may not be symmetrical. The excitation electrodes and the detection electrodes can each be mis-aligned. Furthermore the excitation voltage can, due to cross-talk, be at least partially coupled to the detection voltage.
The present invention seeks to provide an arrangement which can provide compensation for errors of the general type discussed above.
According to this invention there is provided a gyroscopic arrangement for measuring angular speed the arrangement comprising a gyroscope comprising at least one mass supported on support means such that the mass can vibrate in a first mode around a first axis and a second mode around a second axis which is inclined relative to the first axis, there being excitation means to excite vibrations in the first mode and detection means to generate an output comprising at least one detection signal representative of the vibrations in the second mode, the vibration of the first mode being coupled to the second mode when the gyroscope is exposed to an angular motion about a third axis inclined relative to both the first and second axes, the arrangement being such that the detection signal contains information about the angular speed about the third axis, wherein the excitation means are adapted to provide at least one electronic signal to the gyroscope, there being means to vary the or each said electronic signal, wherein, in order to obviate the effect of any errors in the gyroscope caused by undesired geometric properties of the gyroscope and/or undesired electric couplings, the arrangement further comprises a simulation model of the gyroscope, which is connected to receive the signal or signals supplied to the gyroscope by the excitation means, the model being adapted to provide an output equivalent to the output of the gyroscope, the output of the model being compared with the output of the gyroscope as the or each electronic signal is varied to provide a difference signal, the difference signal being provided to an estimator which is also part of the arrangement, the estimator being adapted to determine the presence of and the size of errors within the gyroscope and in response to that determination to adjust the model and to estimate a signal corresponding to the angular speed around the third axis, the estimator and model being adapted to perform an iterative process leading to a situation where the model substantially simulates the instantaneous operation of the system, so that the estimated value of angular speed as estimated by the estimator is substantially the same as the actual angular speed.
Preferably, the estimator comprises an adaptive filter.
Conveniently, the adaptive filter is at least one Kalman-filter.
Advantageously, a signal generator generates a plurality of said electronic signals which are fed to the system and to the model.
Preferably, means are provided to vary the frequency and/or magnitude of signals generated by the signal generator over a period of time.
Conveniently, the gyroscope and the model each produce an output representative of vibration of the mass about the first axis and vibration of the mass about the second axis, the measured output values from the gyroscope and the calculated output values being subtracted, the subtracted values being supplied to the estimator.
Preferably, the subtracted values are passed to discriminators and when the subtracted values are each less than a predetermined value an output signal is generated to indicate that the then estimated value of the said angular speed is substantially correct.